Updated September 2, 2011
About 20% of untreated water samples from public, private, and monitoring wells across the nation contain concentrations of at least one trace element, such as arsenic, manganese and uranium, at levels of potential health concern, according to a new study by the U.S. Geological Survey.
"In public wells these contaminants are regulated by the U.S. Environmental Protection Agency, and contaminants are removed from the water before people drink it," said Joe Ayotte, USGS hydrologist and lead author on the study. "However, trace elements could be present in water from private wells at levels that are considered to pose a risk to human health, because they aren’t subject to regulations. In many cases people might not even know that they have an issue."
Trace elements in groundwater exceed human health benchmarks at a rate that far outpaces most other groundwater contaminants, such as nitrate, pesticides, and volatile organic compounds (VOCs). Most trace elements, including manganese and arsenic, get into the water through the natural process of rock weathering. Radon, derived from naturally occurring uranium in aquifers, also occurs frequently at high levels in groundwater. Human activities like mining, waste disposal, and construction also can contribute to trace elements in groundwater.
Arsenic, uranium, and manganese, were the trace elements in groundwater that most frequently exceeded USEPA human-health benchmarks. Arsenic was found above the USEPA human health benchmark in 7% of wells. Uranium was found in 4% above the human health benchmark, and manganese was found in 12%. Long-term exposure to arsenic can lead to several types of cancer, and high levels of uranium can cause kidney disease. In doses similar to some of those found in this study, manganese can adversely affect child intellectual function and, in large doses, acts as a neurotoxin, causing symptoms similar to those experienced by sufferers of Parkinson’s disease. Radon, a product of the decay of natural uranium, also exceeded its proposed EPA maximum contaminant level in 65% of wells tested (300 Picocuries per liter).
Climate and land use are important factors in trace element distribution. Differences in the concentration of trace elements are related to the climatic conditions and land use of the area. Drier areas of the United States saw higher concentrations of trace elements in groundwater than humid regions. Meanwhile, wells in agricultural areas more often contained trace elements than those in urban areas. However, wells in urban areas contained concentrations of trace elements that more often exceeded human health benchmarks.
Basic geology and geochemistry of water samples helps to predict occurrence of trace elements in groundwater. Redox conditions (related to dissolved oxygen) and pH seem to affect which trace elements persist in groundwater. For example, aluminum and manganese occurred more often in samples that were characterized as anoxic and had slightly acidic pH. Anoxic and slightly alkaline samples commonly contained arsenic and molybdenum. Uranium in groundwater occurred over a wide range of pH and redox conditions because of its association with other compounds. Additionally, samples from glacial and non-glacial sand and gravel aquifers consistently had more occurrences of trace elements in groundwater than samples from other aquifers.
The effects of mixtures of trace elements are poorly understood and could cause further health concerns. Further analysis of the data showed that about one-fifth of wells had exceedances of human health benchmarks and that, of those, about 10 percent actually contained two or more trace elements exceeding human health benchmarks. This raises additional concerns because contaminants can act together to be more toxic than each individual contaminant.
These findings are based on over 5,000 samples collected primarily from public and private wells nationwide. This study is part of efforts by the U.S. Geological Survey's National Water-Quality Assessment Program to monitor the quality of the nation’s groundwater and surface water. Details can be found online.
Human health benchmarks used in this study include U.S. Environmental Protection Agency Maximum Contaminant Levels for regulated contaminants and Health Based Screening Levels (HBSLs) for unregulated contaminants. HBSLs are unenforceable contaminant threshold guidelines developed by the USGS in collaboration with EPA, and the New Jersey Department of Environmental Protection. and Oregon Health Sciences University.
Treated drinking water from public wells is regulated under the Safe Drinking Water Act. Water utilities, however, are not required to treat water for unregulated contaminants. The EPA uses USGS information on the occurrence of unregulated contaminants to identify contaminants that may require drinking-water regulation in the future.